Russian President Vladimir Putin raised the alert status for his nuclear forces to "special regime of combat duty" as Russia and Ukraine battle for the fourth day. 

Putin ordered the Russian defense minister and the chief of the military's General Staff to put the nuclear deterrent forces in a "special regime of combat duty." The move signals tensions could boil over into a nuclear war. 

"He is right now threatening a nuclear escalation," said former DIA intelligence officer Rebekah Koffler. "This a veiled threat – or maybe such a veiled threat - he just met with his chief of general staff and minister of defense. He is escalating the conflict into the nuclear domain in order to de-escalate – that is topple Kyiv’s regime quickly."

Putin spoke at a meeting Sunday with his top advisers and said NATO powers had made "aggressive statements" and that the West was imposing hard-hitting financial sanctions against Russia and himself. 

"Western countries aren’t only taking unfriendly actions against our country in the economic sphere, but top officials from leading NATO members made aggressive statements regarding our country," Putin said in televised comments.

U.S. Ambassador to the United Nations Linda Thomas-Greenfield said Sunday on CBS's "Face the Nation" that Putin's order is an unacceptable escalation. 

"It means that President Putin is continuing to escalate this war in a manner that is totally unacceptable and we have to continue to stem his actions in the strongest possible way," she said. 

Putin warned last week that countries who interfere with his invasion of Ukrainewill face "consequences you have never seen."

Fox News